This invention relates in general to the recapture and utilization of low-grade energy generated in an industrial process, which energy is usually discarded. More particularly, low pressure vapors generated from waste heat in the industrial process are recompressed by a screw compressor with the unique capability of handling wet and contaminated vapors, and returned to a staged evaporation system in accordance with the requirements of the system.
There are many processes in the chemical, petroleum, food and paper industries in which steam, usually at low pressure and often contaminated, is generated in large quantities. Discarding such steam as by venting it to the environment, for example, is not only wasteful of the energy, but undesirable because of air pollution problems. Likewise, condensing the steam and delivering the condensate to a treatment plant for disposal is equally undesirable because of added capital and operating costs in addition to the energy waste. By mechanical vapor recompression, this waste low-pressure steam or other vapor, or low-grade heat which can be used to produce low-pressure steam, can be recompressed to raise the pressure to a useful process level.
High recovery efficiency is attained because most of the energy contained in the vapor is already available to the compressor as latent heat, and thus only a small amount of additional work has to be done on the steam to raise its pressure to the desired level.
Various compressor drives, such as reciprocating engines, gas turbines, steam turbines, or electric motors may be used. With gas turbine or engine drives, optional heat recovery boilers or heat exchangers may be used to provide additional high-pressure steam and hot water for increased system efficiency.
The present invention is concerned generally with the utilization rather than the waste of such energy in a broad range of industrial evaporation processes, but will be described in connection with one of the more challenging operations, namely, the rendering of animal byproducts in food and allied industries.
Rendering operations, as currently practiced, involve processing animal byproducts and converting them to tallow and meatmeal. The tallow is used by the chemical industry in manufacturing primarily fatty acids and other chemicals used in the soap industry. Edible meatmeal, on the other hand, is recycled back to the animals as part of their nutritional diet.
Each raw material, whether it be beef, poultry, fish, blood, etc., requires slightly different processing but, in general, an indirectly heated cooker is used to heat the raw material to evaporate the water. In most cases, clean steam at around 125 psig is used as the heat input to the cookers. Cookers may be either batch-operated or continuously operated, most new cookers being of the continuous type. Typical steam flow requirements for continuous cookers range from 8000 to 20,000 lb/hr.
In a typical continuous rendering process, raw materials, after removal of metal particles, are ground and then sent to the cooker where they are heated by steam jackets to evaporate the water. The evaporated water (contaminated steam) contains some foams and may contain ammonia, fat aerosol (0.05 percent), dimethyl sulfide, ketones, meat particles, feather particles, volatile organics, carried over sand particles, fine metals, and air (from startup operation mainly). The condensed water pH lies generally between 6.5 and 7.5 with a BOD level ranging from 1500 to 2000 and a total suspended solids range 200 to 800 ppm (TSS). The cooking operation may last several hours depending on the nature of raw materials. For processing meat or pork, 3 to 5 hours residence time is needed. Poultry needs 2 hours and blood 8 to 10 hours. It should be noted that as the water is boiled off, the remaining material boils at a higher temperature, often referred to as boiling point elevation. For example, for beef byproducts, a temperature of 65.degree. F. above the initial boiling point temperature of 212.degree. F., or 277.degree. F. is required to drive off the last few percent of moisture.
After most of the moisture is removed, the processed materials are pressed and centrifuged to make the tallow. If necessary, the meatmeal goes through further grinding after the tallow has been removed.
Removing moisture in the manner described is an inefficient process. It typically takes approximately 1.5 pounds of clean boiler steam for each pound of water removed in a single stage evaporation system. The bulk of this energy (approximately 67 percent) is carried away as contaminated steam.
Multiple effect evaporative processes have been tried in an attempt to reduce the energy requirements. In rendering operations, these have generally been two-effect systems which use a sub-atmospheric preconcentrator stage heated by the low pressure contaminated steam. This technique requires a complex recirculation scheme where hot tallow is used to help recirculate the product from one stage to the other. Such techniques as recirculating the hot tallow are complex and not truly energy-efficient. The energy cost savings from this approach have usually been more than counterbalanced by increased electric energy costs associated with driving recirculation pump motors and other accessories.
The general object of the invention is the saving of energy in practicing industrial evaporation processes in which quantities of low-grade energy are generated.
Another object of the invention is the reduction of capital and operating costs in industrial evaporation processes.
Still another object of the invention is the reduction of environmental pollution discharge streams and equipment resulting from contaminated fluids generated in industrial evaporation processes.
A more particular object of the invention is to improve the efficiency of rendering operations.
Yet another particular object of the invention is the simplification and reduction of costs of rendering operations.